It was once the "twin star" of the Earth, but it has a completely different fate

Venus is a terrestrial planet in the solar system, with an average distance from the sun of about 0.72 AU (108 million kilometers) and an orbital period of 224.71 days. Venus rotates from east to west and has the longest rotation period (243 days) among the eight planets. Among the eight planets in the solar system, Venus is called the "twin planet" of the Earth. Due to its similar size and density to the Earth, people speculate that the initial material composition of Venus is also similar to that of the Earth, but based on the only detection data, no conclusion can be drawn at present.

Size comparison between Venus and Earth

Now, the surface environment of Venus is completely different from that of the Earth: the atmospheric pressure is 92 times that of the Earth's surface; the ground temperature is about 465°C and is relatively uniform across the globe; cars will melt on the surface of Venus, making it very unsuitable for life. Venus has a thick atmosphere, mainly composed of carbon dioxide (96.5%) and nitrogen (3.5%). There are many corrosive acidic gases in the clouds about 50 kilometers high.

However, 3.6 billion years ago, Venus was also in the habitable zone of the solar system. Did Venus have oceans and life at that time? What caused Venus to embark on an evolutionary path that was completely different from that of the Earth and become the purgatory it is today? Is Venus the past or the future of the Earth? These questions are the most critical scientific issues in Venus exploration.

The surface of Venus is surrounded by a dense atmosphere, making it difficult to see the true surface from orbit.

Part 1

Exploring Venus

Human exploration of extraterrestrial planets began with Venus. Since the Soviet Union's first Venus exploration in 1961 to the end of 2021, countries around the world have launched a total of 43 Venus probes, including 33 by the Soviet Union (15 successful), 8 by the United States (6 successful), 1 by the European Union (successful), and 1 by Japan (successful). The mission types are mainly Venus flyby, orbit and atmospheric entry exploration missions (33 times), while landing missions (8 times) and floating missions (2 times) are relatively rare. The missions were concentrated during the US-Soviet space race in the 1960s and 1970s. Since the 21st century, there have been only two missions, namely the European Space Agency's Venus Express (Venus Express; 2006-2014) and Japan's Akatsuki (Akatsuki; successfully entered orbit in 2015 and has been in orbit ever since). Akatsuki is currently the only Venus probe in orbit.

There are two main ways to conduct exploration of Venus: ground-based observation and Venus probe. Ground-based observation is an important supplement to the probe mission, which can observe atmospheric composition, atmospheric dynamics and Venus geology. Venus probes are divided into three types: orbital exploration, airborne exploration and ground-based exploration. Orbital exploration is relatively the most mature technology, and orbiters account for the absolute majority of existing Venus missions. Airborne exploration is carried out through air platforms (such as floating balloons) and descending probes/sondes. Ground-based exploration lands on the surface of Venus to obtain information such as the composition of Venus' surface materials, the interaction between the atmosphere and the surface, and earthquakes. However, the harsh environment on the surface of Venus is the main constraint on the survival time of the probe. Based on the survival time, ground-based probes are also divided into short-life workstations (24 hours) and mobile Venus rovers (> 24 hours).

Akatsuki Venus Probe

The advantage of Venus orbiters is that they can observe the atmosphere and space environment of Venus for a long time and on a large scale. Since the 21st century, thanks to the success of ESA's Venus Express and Japan's Akatsuki, people have obtained a large amount of precious remote sensing data of Venus and have a deeper understanding of the atmosphere and climate of Venus.

Venus Express's detection is mainly focused on the cloud layer and the middle and upper atmosphere. It uses a different orbit design from the Pioneer-Venus Orbiter, which allows it to enter different areas of Venus' induced magnetosphere and obtain many new observations and discoveries. Some highlights of Venus Express include: building a global atmospheric circulation model for Venus; drawing a global surface temperature map of Venus, the thermal profile and thermal structure of Venus' atmosphere, and the atmospheric chemical composition profile (CO, SO2, OCS, D/H ratio, etc.), discovering new atmospheric components (O3 and OH); discovering that the average wind speed in the low-latitude region of Venus continues to increase; the existence of a low-temperature layer in the upper atmosphere; the characteristics of atmospheric gravity waves caused by terrain and the characteristics of deep clouds, etc. Venus Express also discovered some suspected active volcanic "hot spots". But overall, Venus Express's detection range and accuracy of the surface are limited.

Venus Express probe to Venus

The exploration target of Japan's Akatsuki focused on the atmosphere and space environment of Venus, and conducted detailed exploration from cloud tops to deep clouds. Akatsuki portrayed the morphology of deep clouds at 35-50 km; discovered large-scale bow-shaped features in the atmosphere of Venus; horizontal jet winds in the middle and low clouds above the equatorial region; obtained the atmospheric temperature distribution above 40 km altitude through radio occultation experiments; discovered the existence of thick clouds with small particles near the transition zone between the upper and middle cloud layers; and discovered the mechanism of thermal tides maintaining the super rotation of the atmosphere through observations and simulations of cloud top thermal tides.

Schematic diagram of the vertical structure of Venus' atmosphere

Part 2

In the atmosphere of Venus

Although the surface of Venus is covered by a thick atmosphere, some electromagnetic waves of specific wavelengths can still penetrate the atmosphere and observe the surface of Venus.

Schematic diagram of the electromagnetic wave band that can penetrate the planet's cloud layer

To date, the most comprehensive and accurate Venus topographic map comes from the radar detection of the US Magellan orbiter.

Venus surface features

The surface of Venus is relatively flat, and the landforms can be divided into three types of geological units: lowlands, plains, and highlands. About 80% of the surface of Venus is covered by smooth volcanic plains, of which 70% have folded ridges and 10% are either smooth or have fractures. Two highlands occupy the remaining 20% ​​of the surface of Venus, one in the northern hemisphere (Ishtar Terra) and the other south of the equator (Aphrodite Terra). The highest mountain on Venus, Maxwell Montes (the highest peak is more than 11 km above the average radius of Venus), is located within the range of Ishtar Terra.

Typical morphological features of the surface of Venus include radial rock walls, shield volcanoes, lava flows, spider web structures, etc.

Only about 940 impact craters remain on the surface of Venus. Interestingly, there are very few impact craters smaller than 30 km, and almost no impact craters smaller than 5 km. At the same time, there is a lack of large impact craters on the surface of Venus. Based on the existing impact craters, it can be inferred that Venus underwent a global surface remodeling event around 750 million years ago. The whole process took about 100 million years. The remodeling event erased the earlier geological records. It is not clear whether this global remodeling event was catastrophic or slow equilibrium.

In addition, there are various tectonic features on the surface of Venus. Active volcanic activity has formed multi-scale tectonic deformations, and eventually formed a global tectonic network associated with volcanoes. The most special of these is a geological unit called tessera. They are isolated blocks of tens of kilometers surrounded by plains, shaped like mosaic floors. Parallel ridges, faults, and grabens in different directions intersect within the tessera, accompanied by a small amount of volcanic activity. The tessera are the oldest terranes that may be preserved on the surface of Venus at present, and may also be related to the role of water. In the future, the tessera will be an important target for Venus exploration.

Rift valleys are similar to Earth's mid-ocean ridges; wrinkle ridges are mainly distributed in lowlands; mosaic blocks span areas of high deformation and may be similar in composition to Earth's continental crust

Compared with remote sensing exploration, landing exploration is more difficult, and the data is less and more precious. The record for the longest survival time on Venus is still held by the Soviet Union's "Venera" series of probes, with the longest record of 127 minutes.

Artist's impression of the Soviet "Vena" lander landing on the surface of Venus (lightning and sulfuric acid rain)

All landers landed in the volcanic plains of Venus. From the photos sent back, it can be seen that there is no liquid water or vegetation on the surface of Venus, only scattered rocks. The landers measured the composition of the surface materials. These composition measurements are not only extremely limited in quantity and have large errors, but also lack data on some key elements (such as sodium). However, these measurements are still the main basis for the composition of Venus materials, especially in the absence of Venus meteorites or returned samples.

Images of the Venus landing site taken by the Soviet Venera 9 and 13 missions

Part 3

Is there "life" there?

Due to its potential similarity to the Earth, whether there is life on the surface or in the atmosphere of Venus has long been a concern of the international academic community. There are two related hypotheses. One hypothesis is that the surface of Venus had a mild climate and a liquid ocean in the early days, until the greenhouse effect gradually got out of control, and all the water evaporated into the atmosphere and escaped. There is no evidence to support this hypothesis; some models indicate that Venus may never have had a liquid ocean. One hypothesis is that there is a habitable zone in the cloud layer of modern Venus, where there are suitable temperature and pressure conditions (~60°C, 1 atmosphere), and micron-sized aerosols have a shielding effect on cosmic rays or ultraviolet rays, which can protect the existence of life.

Hypothesis of habitable environment on Venus surface

(Left) Schematic diagram of the hypothesis that Venus might have been a habitable planet before the greenhouse effect got out of control.

(Right) Schematic diagram of the hypothesized cycle of thermophilic-hyperacidophilic microorganisms living in the cloud haze layers of Venus.

In September 2020, a research team published a paper in the journal Nature Astronomy, announcing that phosphine (PH3) was detected at a certain altitude in the cloud layer of Venus using a ground-based radio telescope, which may be indirect evidence of the existence of life, causing a huge sensation and controversy. The focus is on the multi-solution nature of the observational data, and even if the signal really comes from phosphine, other non-life sources cannot be ruled out. In any case, these new detections and controversies mark Venus as a new hotspot for international planetary and space life detection and research, and an important area of ​​international scientific and technological competition.

Although Venus exploration has gone through many years, it is still in the stage of accumulating key data, and there are many observation gaps that need to be filled. For example, 99% of the mass of Venus' atmosphere is concentrated in the troposphere, especially below 28 km, but there is currently a lack of direct detection data for the deep atmosphere of Venus from the surface to an altitude of 12 km. Radar-based detection of Venus's topography has been stagnant since the Magellan mission, and the existing Venus radar detection resolution is on the order of hundreds of meters, with an accuracy only equivalent to the Mars mission in the 1970s. It is impossible to achieve more subtle identification and classification of Venus's landforms, especially the inability to analyze and study the geological process scale of the surface of Venus, which seriously restricts the understanding of key areas on the surface of Venus and the geological evolution of Venus. In-situ detection of Venus' atmosphere (especially atmospheric element isotope measurement) and fine remote sensing detection of mosaic plots and even in-situ detection have put forward clear and urgent needs for future Venus exploration missions.

Achievable spatial resolution of radar detection by future Venus missions compared to Magellan

Part 4

Let's go to Venus

In June 2021, NASA and ESA approved new missions to Venus, namely the VERITAS, DAVINCI+ and EnVison missions. In addition, Russia and India have also proposed and actively promoted their own Venus exploration missions. International Venus exploration and research is about to usher in a new wave of enthusiasm, and the mystery of Earth's twin planet will be gradually unveiled.

The "Veritas" mission and the "Da Vinci+" are two highly complementary missions, scheduled to be launched around 2030. The full name of the "Veritas" mission is the "Venus Emissivity, Radio Science, Interferometric Synthetic Aperture Radar, Topography and Spectroscopy" mission. The main scientific goal is to generate high-resolution global topographic maps and images of Venus, and to produce a series of global atlases of Venus, including deformation, surface composition, thermal emission and gravity field maps. It attempts to detect whether Venus has an ancient water environment and whether the current volcanic activity is limited to the mantle plume region or has a wider distribution. The full name of the "Da Vinci+" mission is the "Venus Deep Atmosphere Rare Gases, Chemistry and Imaging" mission. Through a descending probe, it directly measures the composition of the atmosphere of Venus during the 63-minute descent, measures rare gases, trace gases and their isotopic composition, and measures the temperature, pressure and wind speed of the atmosphere of Venus. Before reaching the ground, the probe will also take images of the mosaic of Venus to explore its origin and structure, volcanic and weathering history.

EnVison is scheduled to launch in 2032 and is an orbital high-resolution radar mapping and atmospheric research mission for Venus. The scientific objectives are to search for active geological processes, measure surface temperature changes associated with active volcanism, characterize regional and local geological features, determine crustal support mechanisms and constrain mantle and core properties. The orbital high-resolution radar mapping and atmospheric research mission for Venus can detect centimeter-level surface changes, characterize volcanic and tectonic activity, and estimate the rates of weathering and surface alteration. The subsurface radar sounder will map faults, strata, and weathering within a depth of about 100 m in the regional subsurface, identifying structural relationships and geological history.

Russia's Venera-D mission concept is in the preparation stage, marking Russia's determination to return to Venus. The Venera-D mission concept has been modified several times, and the current baseline mission consists of an orbiter and a short-lived (2-3 hours) Vega-style lander. In addition to the baseline mission, a series of potential elements are under discussion, such as balloons, sub-satellites, and long-lived (24 hours) ground stations. It is also proposed to return samples from Venus in three batches from 2029 to 2034.

In addition, other interplanetary exploration missions can also explore Venus during the Venus gravity assist phase. These missions include NASA's Parker Solar Probe, ESA's Solar Orbiter and Jupiter Icy Moon Explorer (JUICE).

Question and Answer Session/

1. The report mentioned that the longest the Venus lander survived was only about two hours. The temperature and pressure on the surface of Venus do not seem to be very extreme. Is the temperature and pressure the main reason for the short survival time of the lander? Are there other reasons?

A: It is mainly due to the surface temperature and pressure of Venus. The environment on the surface of Venus is a bit like a pressure cooker. When working, electronic devices and scientific instruments must first be cooled to ensure that they do not overheat, but this is very difficult in the environment of Venus. When exploring Mars, some instruments that work for a long time usually choose to work at night and do not need additional cooling methods. In contrast, the continuous high temperature on Venus puts higher requirements on the design of electronic devices and circuits.

2. Venus is about the same size as Earth, so why does Earth have a moon but Venus doesn’t?

A: This is a good question, but we don't know yet. The exact origin of the moon is still unclear, but the most accepted one is the giant impact origin - in the early stage of the formation of the primitive earth, it was hit by a celestial body as large as Mars, and then re-evolved to form the current earth and moon. Venus may not have experienced such a process, which may also have a great impact on the evolution of Venus. Compared with the earth, Venus may be a negative example, that is, without a large natural satellite, whether it will take another evolutionary path.

3. When did the surface of Venus become so hot? Why did Venus have no oceans in the beginning?

A: We don't know the answer to this question either, and it's still speculation. We speculate that Venus was probably very similar to Earth when it was first formed (in the first 100 million years). But it's not clear whether this is actually the case. We don't see any traces of an ocean on the surface of Venus today. But it's also possible that the global magma coverage about 700 million years ago erased some traces of the early ocean. This is also a problem that future Venus exploration missions want to solve.

4. Is our country preparing a plan to explore Venus?

A: We are currently conducting some preliminary research to help with future exploration plans. If China wants to explore Venus, what should it explore and what preparations should it make. These are mainly scientific ideas. But there is no specific Venus exploration mission yet.

5. Is there any preference, such as a lander or a floating platform?

A: With current technology, landing and floating platforms are more difficult. Keeping the lander and the scientific instruments on board alive and working requires very high design requirements for the lander and scientific payload. If it is an aerial platform, there are also many requirements for the aerostat or aircraft itself. So in the next decade, if China wants to explore Venus, it may still consider orbiters as the main method, but it can consider carrying some unique exploration instruments to do exploration that other countries have not done or achieved.

About the speaker

Dr. Zhao Yuyun is an associate researcher at the Lunar and Planetary Science Center, Institute of Geochemistry, Chinese Academy of Sciences, and the Center for Excellence in Comparative Planetology, Chinese Academy of Sciences. He is a member of the preliminary scientific research team for China's first Mars exploration mission. He has been engaged in the study of planetary geological environment evolution for a long time. In recent years, he has paid attention to Venus exploration and participated in the preliminary research and demonstration of scientific issues of Venus exploration.

Source: China National Astronomical Service